Pharmacometabolomic signatures as individualised biochemical response evidence: towards a precision jurisprudence framework for forensic toxicology and criminal responsibility.

Precision medicine is transforming drug response from a population level assumption into an individualised biochemical event. Pharmacometabolomics, which profiles metabolic signatures associated with drug exposure and response, offers a mechanistic basis for explaining interindividual variability in therapeutic efficacy, adverse drug reactions, neurocognitive impairment and toxicological ambiguity. Legal systems, however, continue to evaluate drug related conduct largely through conventional toxicology, generalised impairment thresholds and binary assumptions about intent, voluntariness and negligence. This Perspective proposes individualised biochemical response evidence, described here metaphorically as an emerging category of biochemical legal identity, as a prospective research and governance framework rather than a validated evidentiary standard. Its primary focus is forensic toxicology and criminal responsibility, with medical negligence, product liability, AI guided prescribing, pharmaceutical regulation and biochemical privacy discussed as secondary implications requiring separate validation. The claim is not that metabolomic profiles are currently capable of establishing individual causation, voluntariness or capacity, but that a disciplined evidentiary pathway, moving from biological plausibility through analytical validity, clinical validity and forensic validity to legal admissibility, would be required before such evidence could inform these determinations. A precision jurisprudence framework is proposed to guide this transition while preserving safeguards against biochemical determinism, privacy intrusion and algorithmic opacity.